Plasma lamps provide extremely bright, broadband light, and are useful in applications such as projection systems, industrial processing, and general illumination. The typical plasma lamp manufactured today contains a mixture of gas and trace substances that are excited to form a plasma. Plasma interaction with the trace substance (Selenium or other) gives rise to light in the UV, visible, and near infrared portions of the electromagnetic spectrum. Gas ionization resulting in plasma formation is accomplished by passing a high-current through closely-spaced electrodes contained within the vessel that is the gas fill reservoir. This arrangement, however, suffers from electrode deterioration due to sputtering, and therefore exhibits a limited lifetime.
Electrode-less plasma lamps driven by microwave sources have been disclosed in the prior art. For example, both U.S. Pat. No. 6,617,806B2 (Kirkpatrick et. al.) and U.S. patent application No. US2001/0035720A1 (Guthrie et. al.) disclose similar basic configurations of a gas fill encased either in a bulb or a sealed recess within a dielectric body forming a waveguide, with microwave energy being provided by a source such as a magnetron and introduced into the waveguide and heating the plasma resistively. U.S. Pat. No. 6,737,809B2 (Espiau et. al.) discloses a somewhat different arrangement whereby the plasma-enclosing bulb and the dielectric cavity form a part of a resonant microwave circuit with a microwave amplifier to provide the excitation.
In each of the embodiments described above, a dielectric or metal/dielectric waveguiding body forming—whether deliberately or unwittingly—a resonant cavity surrounding the bulb containing the plasma is used. The driving microwave energy is introduced into the waveguide body using various probing means well-known to those skilled in the art of microwave engineering. The waveguide body surrounding the bulb brings with it a host of difficulties including wasted light, lamp size related to resonance or excitation frequency, manufacturing obstacles, and related costs. These obstacles are overcome by the approach presented herein.